Steering motors power supply method and power supply system adapted to such a method

ABSTRACT

A method is provided for controlling power supply of an electrically powered steering system on-board an automotive land vehicle equipped with a high voltage source and a low voltage source. A first and a second steering motor operate at a nominal operating voltage which is higher than the voltage of the low voltage source, are fed by the high voltage source and may absorb respectively a first and a second nominal operating power. A step-up circuit is connected to the low voltage source, and is adapted to deliver a back-up electrical power in case of failure of the high voltage source. In the method, in case of failure of the high voltage source, back-up electrical power absorbed by each of the two motors is limited on the basis of a priority rule.

BACKGROUND AND SUMMARY

The present invention relates to a method for controlling power supplyof two electrically driven steering motors on-board an automotivevehicle. The invention also concerns a power supply system adapted toperform such a method.

Some vehicles are equipped with two power networks of differentvoltages: a high power network and a low power network each equippedwith their own power storage system such as a battery. Differentelectrical systems are connected to one or the other of said networks,for example depending on their power requirements.

This is most frequent on electrically driven vehicles or hybrid electricvehicles where it is known to have a high voltage traction powernetwork, which feeds a traction motor, and a low voltage network, whichfeeds conventional vehicle equipment systems such as the lights, theelectronic control system of the vehicle, power window motors,windshield wiper motors, etc.

From document JP-A-2007-1324, it is known that a steering assistancesystem comprising an electric motor may be connected to the high voltagepower network of the vehicle, in such case the traction power network.From that document, it is also known to provide a step-up circuitconnected to the low voltage source, said step-up circuit being adaptedto deliver a back-up electrical power in case of failure of the tractionhigh voltage source.

Some vehicles have two steering axles. In order to operate thosesteering axles, each of them may be provided with its own electricsteeling system. Each electric system can be a full electric system,where the electric system operates the steering system on its own asknown steer-by-wire systems, or a more conventional steering assistancesystem where the electric system operates in parallel to a mechanicalsystem to alleviate the effort which the driver needs to put in themechanical system to operate the steering system. It can be providedthat each electric steering system is provided with an electric steeringmotor, and it can be provided that each motor is fed by the high voltagenetwork. The electric steering system, whether a full electric system ora steering assistance system, can operate directly on the steeringmechanism, for example with a steering motor engaging the mechanismdirectly through a rack and pinion mechanism, or indirectly, with asteering motor driving a hydraulic pump which feeds a hydraulic steeringactuator.

In case of failure of the high voltage network, it can be imagined thata back-up system such as the one described in JP-A-2007-1324 can be usedto deliver back-up power to the two steering motors. But in such a case,available power will be reduced, and each motor will not be able tooperate in the same way as in normal use, and that is even more truewhen two steering motors need to be fed. Therefore, an optimized controlof the operating conditions of the two motors is needed, in order forthe steering system to remain operational despite a reduced availableelectric power.

This invention aims at proposing a method for controlling power supplyof two electrically driven steering motors on-board an automotive landvehicle, which manages priority between the two motors and allows themto operate properly with a reduced electrical power.

To this end, the invention concerns a method for controlling the powersupply of an electrically powered steering system on-board an automotiveland vehicle equipped with a high voltage source and a low voltagesource, wherein a first steering motor operates at a nominal operatingvoltage which is higher than the voltage of the low voltage source, isfed by the high voltage source and may absorb a first nominal operatingpower, and wherein a step-up circuit is connected to said low voltagesource, and is adapted to deliver a back-up electrical power in case offailure of said high voltage source, characterized in that the steeringsystem comprises a second steering motor which operates at a nominaloperating voltage which is higher than the voltage of the low voltagesource, which is fed by the high voltage source and may absorb a secondnominal operating power, and in that, in case of failure of the highvoltage source, back-up electrical power absorbed by each of the twomotors is limited on the basis of a priority rule.

Thanks to the invention, each motor absorbs a limited power depending onthe priority rule. This allows reducing the global power absorbed by thetwo motors in case of failure of the traction network and, as aconsequence, to downsize the back-up network.

According to further aspects of the invention, which are advantageousbut not compulsory, such a method may incorporate one or several of thefollowing features:

-   -   in case of failure of the high voltage source, the first        steering motor is controlled to absorb no more than a first        percentage of its nominal operating power, the second steering        motor is controlled to absorb no more than a second percentage        of its nominal operating power, and the first percentage is        higher than second percentage;    -   in case of failure of the high voltage source, the motors are        controlled so that a first fraction of the back-up power is        absorbed by the first steering motor, so that a second fraction        of the back-up power is absorbed by a second motor, and so that        the first fraction is larger than the second fraction;    -   the priority rules varies according to at least one vehicle        parameter.    -   the rotation speed of each of the motors is controlled, in case        of failure of the traction high voltage source, so as to be able        to generate a torque capacity with a value equal to the torque        value which it is capable of delivering in a normal        configuration of said high voltage source;    -   the high voltage source is connected to a high voltage power        network of the vehicle to which is connected a traction motor        capable of delivering driving torque to a driveline of the        vehicle; and    -   the low voltage source is connected to a low voltage power        network of the vehicle to which are connected low power        consumers of the vehicle.

The invention also concerns a power supply system for an electricallypowered steering system on-board an automotive land vehicle, equippedwith a high voltage source and a low voltage source, wherein a firststeering motor operates at a nominal operating voltage which is higherthan the voltage of the low voltage source, is fed by the high voltagesource and may absorb a first nominal operating power, and wherein astep-up circuit is connected to said low voltage source and is adaptedto deliver a back-up electrical power in case of failure of saidtraction high voltage source, characterized in that the steering systemcomprises a second steering motor which operates at a nominal operatingvoltage which is higher than the voltage of the low voltage source,which is fed by the high voltage source and may absorb a second nominaloperating power, and in that the power supply system comprises means tocontrol the rotation speed of each of the motors so as to, in case offailure of the traction high voltage source, limit the electrical powerabsorbed by each motor on the basis of a priority rule included in adataset.

According to further aspects of the invention which are advantageous butnot compulsory, such a power supply system may incorporate one orseveral of the following features:

-   -   the means to control the rotation speed of each motor comprise        two inverters, each connected to one motor, and a power        management unit connected to the two inverters;    -   a first motor delivers torque to a front axle steering        assistance system of the vehicle and a second motor delivers        torque to a rear axle steering assistance system of the vehicle;    -   the control means include a memory where the dataset is stored        and the control means have access to said memory; and    -   the dataset (D) may be fixed or adjustable.

BRIEF DESCRIPTION OF THE DRAWINGS

The invention will now be explained in correspondence with the annexedfigures and as an illustrative example. In the annexed figures:

FIG. 1 is a diagram of a power supply system according to the invention;

FIG. 2 is a torque versus rotation-speed chart representing theoperation conditions of two motors with which a method according to theinvention can be performed.

DETAILED DESCRIPTION

The invention will be hereinafter be described in the context of ahybrid electric automotive vehicle, such as a truck T, which is providedwith two steering axles, each being equipped with a steering assistancesystem driven by an electrically fed steering motor. A front axlesteering assistance system 14 is equipped with a front steering motor 0and a rear axle steering assistance system 24 is equipped with a rearsteering motor 20. Each of the front motor 10 and rear motor 20 is forexample an alternating current motor and is therefore respectivelyconnected to a power supply system S via a three phase inverter,respectively 12 and 22. Inverters 12 and 22 are adapted to control therespective rotation speeds ω<< > and ω20 of steering motors 10 and 20.The front and rear motors may have the same nominal power rating or mayhave a different nominal operating power ratings. The nominal operatingpower rating can be considered as the maximal electrical power which themotor may absorb under normal operating conditions.

The power supply system S is provided with an electronic powermanagement unit 30 which is adapted to communicate with the twoinverters, in particular to provide them with electronic control signalsS12 and S22. A memory 32 is integrated in the power supply system. Thismemory is adapted to contain a data set D. Memory 32 can be implementedas a separate component. Alternatively, it may be integrated into thepower management unit 30.

Each of inverter 12 and inverter 22 is connected, via a high voltageline 40, with a high voltage power network 42, to which a high voltagebattery set 44 is connected. In this example, the high voltage networkis a traction network to which a traction motor of the vehicle isconnected. The nominal voltage of the traction network can be comprisedbetween 250 and 1000 volts. Nevertheless, the high voltage network couldhave a lower nominal voltage, for example comprised between 30 and 100volts.

A back-up electrical power network 46 is also connected to the twoinverters. This network 46 is connected to a low voltage source, whichcan be a low voltage battery set 48 pertaining to a low voltage network.A step-up circuit 50 is connected to low voltage battery set 48 and tothe high voltage line 40. The step-up circuit 50 is used to raise thevoltage of the electrical power delivered by low voltage battery set 48to high voltage line 40, when necessary. The low voltage network canfeed low power systems such as the lights, the electronic control systemof the vehicle, power window motors, windshield wiper motors, etc. . . .It's nominal voltage can for example be in the order of 12 to 24 volts.

The step-up circuit 50 may be provided with one or several transformers.Alternatively, the step-up circuit could be a dc-dc boost converter. Itsoutput voltage can be set at various levels.

The high voltage line 40 comprises a diode, which allows passage ofelectrical current from high voltage battery set 44 to inverters 12, 22.Step-up circuit 50 is connected to high voltage line 40 by a back-upline joining the high voltage line 40 between diode and the inverter 12,22. Back-up line includes a diode which allows passage of electricalcurrent from step-up circuit 50 to high voltage line 40.

A capacitor may be connected in parallel to inverters 12, 22, betweenthe two cables of line 40 which, in the example shown, are connected tothe anode and to the cathode of the high voltage battery set 12. The twocables of back-up line are respectively connected to the anode and tothe cathode of the low voltage battery 48.

The diodes are respectively located on cables of the high voltage lineand of the back-up lines so as to allow current to flow only in thedirection from the anode of the respective battery towards theinverters. The anode cables of both lines are connected downstream ofboth diodes.

In normal use, steering motors 10, 20 are electrically fed by highvoltage power network 10. A failure of traction power network 42 mayoccur. Such a failure can be provoked by a failure of a fuse, a flaw ofa relay of battery set 44 or another problem. Failure of the tractionpower network can be total, whereas the available voltage and power arereduced to zero, or only partial, whereas the available voltage and/orpower may be reduced compared to their nominal rating. Partial failureof the battery can be the consequence of the temperature of the batteryfalling below an optimal range of temperature. In both cases, thevoltage and/or the power available from the traction power network 42decreases to such an extent that steering motors 10, may not beelectrically properly fed anymore and this could lead to a loss ofcontrol of the vehicle by the driver.

Therefore, in case of failure of the high voltage network 42, back-uppower network 46 automatically delivers electrical power to inverters 12and 22, in order for steering motors 10 and 20 to operate properly.

As back-up power network 46 is not able to deliver enough electricalpower to operate the two steering motors in the same conditions as innormal configuration, when the high voltage network is working properly,a priority rule is considered to operate the two motors, in fact, thepower absorbed by the two motors is limited on the basis of thispriority rule: priority is given to one steering system, for example tothe front steering assistance system 14, in order to allow the driver ofthe vehicle to keep a good control of his path.

One example of such priority rule is to provide that, in case of failureof the high voltage source, the front motor may be controlled to absorbno more than a first percentage of its nominal operating power, thesecond motor may be controlled to absorb no more than a secondpercentage of its nominal operating power, and to provide that the firstpercentage is higher than second percentage. For example, while bothmotors may operate at 100% of their nominal operating power when thehigh voltage network is operative, it can be provided that, in case offailure of the high voltage source, the front motor may operate at nomore than 40% of its nominal rating, while the rear motor may notoperate at more than 25% of its nominal power.

Another way to define a priority rule is to control the motors so that,in case of failure of the high voltage source, a first fraction of theback-up power is absorbed by the first motor, so that a second fractionof the back-up power is absorbed by the second motor, and so that thefirst fraction is larger than the second fraction. As an example, it canbe defined that X % of the back-up electric power Ptot available for thesteering system may be allocated to one steering motor and (100−X) % tothe other steering motor 20, where X is between 51 and 100. Priority isgiven to the operation of the first steering system when X is strictlylarger than 50. It can for example be decided to devote 60% of theback-up electric power Ptot available for the steering system to thefront motor and only 40% to the rear motor. Such priority rule will beadapted to cases where both motors have a similar nominal operatingpower rating.

In a refined embodiment of the invention, the priority rule variesaccording to at least one vehicle parameter such as the vehicle speed,the vehicle load, the vehicle axle load sharing ratio, the vehicleoperating location as determined by a positioning system, the value ofavailable back-up electrical power, etc. . . . For example, it may beprovided that the front motor receives 60% of the available back-uppower for the steering system when the vehicle is running above acertain speed threshold, and 80% when the vehicle is running below saidspeed threshold. The variation may by steps, or may be progressive, forexample linear with a predetermined parameter, or may be a more complexfunction of several vehicle operating parameters.

The elements which define the priority rule are stored in dataset D,which can therefore be fixed or adjustable depending on whether avariation of the priority rule is provided.

In order for each of motors 10 and 20 to be able to deliver, in case offailure of high voltage network 42, a torque capacity equal, or at leastas close as possible, to the torque capacity that they deliver in normaluse, their respective rotation speeds ω10 and ω20 may be controlled. Astorque capacity is proportional to power and inversely proportional tothe rotation speed of a motor, if available power is reduced, rotationspeed has to be reduced in order for the torque capacity to remainconstant.

As shown on FIG. 2, operation conditions of each of motors 10 and 20 arecontrolled so as to respect absorbed power limitations curves Plim1 andPlim2, derived from dataset D included in memory 32.

In case of high voltage network 42 failure, and in order for torquecapacity Ci0 of steering motor 10 to match the torque capacity in normaluse, the rotation speed of the steering motor 10 is reduced, following aconstant power curve Plim1. To this end, electronic power managementunit 30 computes the right rotation speed ωιo for steering motor 0 andcontrols the inverter 12 via electronic signal S12. The constant powervalue used to determine the rotation speed reduction is given by thepriority rule included in dataset D. In normal use, operation conditionspoint of motor 10 is referenced as couple (ω0, Co) on FIG. 2. In case offailure of high voltage network 42, operation conditions point is movedto match curve Plim1 at another point (ω0, C0). Torque CIO can bemaintained at a constant value C0 while rotation speed ω10 is reducedfrom ω0 to ωF.

This rotation speed reduction is also done for motor 20 delivering atorque C20. Rotation speed ω20 of rear steering motor 20 is reduced onthe basis of a constant power curve Plim2 corresponding to the amount ofback-up power, say 20%, which is not used by steering motor 10. Toexecute this operation, electronic signal S22 is sent by powermanagement unit to inverter 22, on the basis of a computation performedby unit 30.

The priority rule permits to downsize back-up power network 46 byreducing the global power absorbed by motors 10 and 20 under suchconditions, while still maintaining enough effectiveness of the steeringsystem, at least for operation according to a so-called limp-mode.

Steering motors 10 and 20 may be AC or DC motors. Steering motors 10 and20 in the example are AC motors which may be synchronous orasynchronous.

Setting a priority rule limits for example the risk or the extent of thefront axle steering assistance system 14 lacking power in case offailure of the high voltage network and thus, preserves drivability ofthe vehicle. The priority rule also permits, by downsizing the back-uppower network 46, to lower the size, the weight and the cost of thisnetwork. As back-up power network 46 is connected to low voltage batteryset 48 which feeds several other powered systems of the vehicle, such asradio, lights or heating, the limitation of the power absorbed by motors10 and 20 from the back-up network 46 may protect at least to a certainextent those other systems from a lack of electrical power.

The invention is applicable with trucks, buses, cars and any otherautomotive land vehicle having steering wheels.

1. Method for controlling the power supply of an electrically poweredsteering system on-board an automotive land vehicle equipped with a highvoltage source and a low voltage source, comprising operating a firststeering motor at a nominal operating voltage which is higher than avoltage of the low voltage source, feeding the first steering motor bythe high voltage source and absorbing via the first steering motor afirst nominal operating power, delivering back-up electrical power via astep-up circuit that is connected to the low voltage source upon failureof the high voltage source, operating a second steering motor at anominal operating voltage which is higher than the voltage of the lowvoltage source, feeding the second steering motor by the high voltagesource and absorbing via the second steering motor a second nominaloperating power, and limiting, in case of failure of the high voltagesource, back-up electrical power absorbed by each of the two motors onthe basis of a priority rule.
 2. Method according to claim 1, wherein,in case of failure of the high voltage source, the first steering motoris controlled to absorb no more than a first percentage of its nominaloperating power, the second steering motor is controlled to absorb nomore than a second percentage of its nominal operating power, and inthat first percentage is higher than second percentage.
 3. Methodaccording to claim 1, wherein, in case of failure of the high voltagesource, the motors are controlled so that a first fraction of theback-up power is absorbed by the first steering motor, so that a secondfraction of the back-up power is absorbed by a second motor, and so thatthe first fraction is larger than the second fraction.
 4. Methodaccording to claim 1, wherein the priority rules varies according to atleast one vehicle parameter.
 5. Method according to claim 1, wherein therotation speed of each of the motors is controlled, in case of failureof the traction high voltage source, so as to be able to generate atorque capacity with a value equal to the torque value which it iscapable of delivering in a normal configuration of the high voltagesource.
 6. Method according to claim 1, wherein the high voltage sourceis connected to a high voltage power network of the vehicle to which isconnected a traction motor capable of delivering driving torque to adriveline of the vehicle.
 7. Method according to claim 1, wherein thelow voltage source is connected to a low voltage power network of thevehicle to which are connected low power consumers of the vehicle. 8.Power supply system for an electrically powered steering system on-boardan automotive land vehicle, equipped with a high voltage source and alow voltage source, wherein a first steering motor operates at a nominaloperating voltage which is higher than the voltage of the low voltagesource, is fed by the high voltage source and may absorb a first nominaloperating power, and wherein a step-up circuit is connected to the lowvoltage source and is adapted to deliver a back-up electrical power incase of failure of the traction high voltage source, wherein thesteering system comprises a second steering motor which operates at anominal operating voltage which is higher than the voltage of the lowvoltage source, which is fed by the high voltage source and may absorb asecond nominal operating power, and the power supply system comprisesmeans to control the rotation speed of each of the motors so as to, incase of failure of the traction high voltage source, limit theelectrical power absorbed by each motor on the basis of a priority ruleincluded in a dataset.
 9. Power supply system according to claim 8,wherein the means to control the rotation speed of each motor comprisetwo inverters, each connected to one motor, and a power management unitconnected to the two inverters.
 10. Power supply system according toclaim 8, wherein a first motor delivers torque to a front axle steeringassistance system of the vehicle and in that a second motor deliverstorque to a rear axle steering assistance system of the vehicle. 11.Power supply system according to claim 9, wherein the control meansinclude a memory where the dataset is stored and in that the controlmeans have access to the memory.
 12. Power supply system according toclaim 8, wherein the dataset is fixed.
 13. Power supply system accordingto claim 8, wherein the dataset is adjustable.